The Histopathology Core will be responsible for the pathologic evaluation and characterization of CMS and visceral necropsy tissues from seronegative and simian immunodeficiency virus (SIV) infected rhesus macaques (Macaca mulatta) acquired through Core B: Experimental Animal Core. The Histopathology Core will dissect, prepare and store tissues for further histopathologic analyses as required by the other projects and cores of "PO1: Drugs of abuse affecting AIDS pathogenesis". For this purpose we will be responsible for the accession, pathologic evaluation, characterization and storage of specimens including the accession, processing, embedding, storage and sectioning of seronegative and SIV infected CNS and visceral tissue samples from rhesus macaque necropsy materials. Specimens will be analyzed by light microscopy using standard histochemical techniques. Tissues will be characterized and graded by the pathological findings according to established classifications. Further, specimens will be subjected to further histopathologic analysis as defined by by the various projects using immunohistochemical techniques for antigen detection and established in situ hybridization techniques for mRNA detection/localization. Specimens will be analyzed and characterized according to our histopathologic findings.